PMT's, Image intensifiers and Electron Bombarded Active Pixel Sensors (EBAPS) are used in various ways. Many such systems are further distinguished by the spectral or spatial information contained in the incoming and output light images. This invention can provide both spectral and spatial information without the need for moving parts. Spatial information is derived by sensing the position at which the photoelectrons exit the photocathode structure. Spectral information is derived by comparing the intensity of detected light between at least two sequential measurements where the cathode of this invention is toggled between spectral states. One example where this invention is useful involves the use of a PMT that is looking for multiple florescent signatures in a flow cytometry system. The lack of moving parts in this invention facilitates very rapid spectral shifts in spectral response to be achieved. This permits system manufacturers to measure the “color” for example of a single cell as it passes through the analysis window of a flow cytometer without the need for multiple PMTs or a complicated optical path.
Another interesting application involves low light imaging systems. Night vision equipment is widely used in military and security applications. A number of sensor technologies are currently used to achieve low light level imagery. This invention is particularly useful for image intensifier based night vision camera systems. In these systems, a photocathode detects incoming light. The photocathode performs a photon to free electron conversion. Intensifiers also contain a gain mechanism by which the energy or number of free electrons are multiplied. Finally, intensified camera systems contain means by which the multiplied signal from the gain stage may be converted into a viewable image such as through video signals. Prior art intensifiers have incorporated a micro-channel-plate (MCP) as an electron multiplier and use a phosphor screen which may be viewed directly or may be coupled to a CCD or CMOS image sensor to generate an ultimate video picture.
The great majority of commercially available night vision systems generate a monochrome image. There are a number of distinct approaches that have been proposed to generate color night vision sensors and cameras. There are calculated approaches (New Scientist Magazine, Issue 2486, 12 Feb. 2005, page 21) that map the monochrome image of a standard image intensifier onto a color pallet for display and, there are approaches that generate information based upon altered lighting or sensor spectral response. This invention is related to the latter class of systems.
There is described a novel photocathode structure in which a rectifying junction is located on the surface of or contained in the first layer of a transmission mode semiconductor photocathode structure. In the event that the photocathode is positioned on a support, light directed at the photocathode layer will first pass through the support layer such as glass and then through a rectifying junction before striking the balance of the photocathode structure. If such a support is not required then the incoming light will strike the photocathode structure directly passing through a rectifying junction before entering into the rest of the photocathode structure. Also described is a novel system in which a change in spectral response can be achieved by reverse biasing the rectifying junction. By comparing the output of the system with and without bias, one is able to create a multi color image. Thus, the novel photocathode makes possible for example high performance color night vision intensifier systems and cameras. Although the spectral distribution of a sensor can also be shifted using an electrically alterable optical filter placed in front of a standard PMT or image intensifier, such filters, as a means toward multi color imaging, cause undesirable signal loss. Notwithstanding, if one remains interested then one of the most suitable filters of this type designed to date is described in NASA tech brief NPO-20245.
In describing the quality or usefulness of a night vision sensor one usually refers to an application specific figure of merit. For example, an ideal head mounted color night vision sensor for use in military environments will have a low weight, detect 100% of the incident photons within it's pass-band with Shot noise limited signal to noise ratio (SNR) while simultaneously drawing virtually no power. Simultaneously, system weight would be minimal.
All existing and proposed color night vision cameras make some compromises vis-à-vis the ideal sensor. The color night vision system proposed by Smith in U.S. Pat. No. 6,570,147 employs multiple image intensifiers positioned behind a prism that splits different colors of light to each sensor. This approach is very efficient in preserving all of the photons incident upon the objective lens and routing them to a suitable sensor. However, this system suffers from size, weight and power problems. The fact that each color channel requires a separate intensifier/video capture channel with its associated size and power penalty, effectively eliminates this as a viable approach for a head mounted sensor. Splitting prism systems also typically require objective lenses that have a large back focal length. This represents a serious limitation for head mounted night vision systems where wide field of view and low F/# lenses are required. These systems have the further disadvantage of being expensive because of their many components.
A number of color night vision cameras that employ a movable filter to sequentially change the spectral response of the night vision sensor have been proposed. These are exemplified in U.S. Pat. Nos. 6,614,606 and 4,724,354. The temporally and spectrally distinct frames are then recombined either in the optical or electrical domain in order to create a color output image. Filter approaches are generally somewhat less efficient with incident photons than those which employ a splitting prism in that spectral changes are affected by absorbing photons before they reach the photocathode. The major draw back of this class of systems is the reliability and size of the moving filters. Reliability and size make these systems unsuitable for use for example in military head-mounted goggles.
A second class of filter-based color night vision goggles generate a color image by filtering small physically adjacent patches of the photocathode in order to generate a mosaic of spectrally and spatially distinct patches at the output of the sensor. These patches, also referred to as tiles, can then be optically or electrically recombined into a color image. An example of this approach is described in U.S. Pat. No. 5,742,115. On first inspection, this appears to be a lightweight, compact and potentially low power approach to achieving a head-mounted color night vision system. Unfortunately, the spectrally tiled photocathode approach suffers from the imperfect modulation transfer function (MTF) found in image intensifiers. Furthermore, these systems suffer from very low manufacturing tolerance budgets; this can result in a prohibitively expensive sensor.
Photocathodes come in a wide variety of types and subclasses. Many of the early night image intensifiers employed Multialkali Antimonide Photocathodes as described by Sommer in Photoemissive Materials, A. H. Sommer, Robert E. Krieger Publishing Company, Huntington, N.Y., 1980. Modern versions of these cathodes account for a significant fraction of the image intensifiers sold and in use today. In the 1950s, research on a new class of photocathodes was anchored and accelerated when William E. Spicer reported in Phys. Rev. 112, 114 (1958) concerning a detailed model as to give understanding and permit engineering of negative electron affinity semiconductor photocathodes. The instant disclosure makes use of negative electron affinity (NEA) and Transferred Electron (TE) semiconductor photocathode structures. After Spicer's publication, numerous varieties of photocathodes were developed. U.S. Pat. No. 3,631,303 details one of the early designs that employs a band-gap graded semiconductor optical absorber layer. In the described structure, the semiconductor substrate is a large band-gap material that acts as a passivation layer for the back surface of the active layer. Though described as a reflection mode photocathode, using a thin substrate window layer, the structure works equally well in a transmission mode. A modern third generation image intensifier photocathode as disclosed in U.S. Pat. No. 5,268,570 makes use of a P-type GaAs or InGaAs optical absorber layer coupled with a P-type AlGaAs window layer. High P-type doping levels typically >1×1018/cm3 and the larger band-gap of the AlGaAs or AlInGaAs window layer result in a hetero-structure that is very efficient at preserving photo-generated electrons. An example and method of manufacture of a modern GaAs photocathode is described in U.S. Pat. No. 5,597,112. Photoelectrons that diffuse to the hetero-junction experience a potential barrier and are reflected back into the absorber layer and hence, toward the vacuum emission surface. The ramped band-gap structure described in U.S. Pat. No. 3,631,303 plays a similar role in directing the diffusion/drift of photoelectrons toward the vacuum emission surface.
These semiconductor NEA photocathodes can be classed as passive photocathodes. In use, these cathodes are set to a single fixed electrical potential. In other words, there are no electric fields within the cathode that are specified through the application of a bias voltage across two or more contact terminals. It turns out that all commercially successful, semiconductor, NEA cathodes that are used for night vision applications fall into this class. In part this results from the fact that it is very difficult to eliminate electric field induced dark current sources in active photocathode structures. Current GaAs based night vision cathodes typically show room temperature emitted dark currents on the order of 1×10−14 A/cm2 while simultaneously demonstrating external quantum efficiencies in excess of 40%; this is a very demanding requirement for biased photocathode structures.
A number of active photocathode structures are known. Some cathode structures such as those described in U.S. Pat. Nos. 3,361,303, 5,047,821 and 5,576,559 have met with commercial success in gated applications. Others, such as that described in U.S. Pat. No. 3,814,993, are not known to have had a significant commercial impact. Active, electrically gated/shuttered intensifier systems are typically used in conjunction with active illumination and are therefore more tolerant of high photocathode dark current. In fact, commercially available active cathodes often show room temperature emitted dark currents on the order of 5×10−8 A/cm2. Dark currents of this order eliminate intensifiers from contention as room temperature passive night vision sensors. The power requirements associated with cooling eliminate the chilled variant of these cathodes as a candidate for small light weight units such as head mounted night vision systems. However, where weight is not a consideration, these photocathodes may be used in such equipment.
In its most basic form, this invention is a hetero-junction semiconductor photocathode containing a larger band gap window layer as part of a rectifying junction, and an optical absorber-emitter layer. The window layer contains a p-type section directly overlying the optical absorber layer. The side of the p-type layer opposite the optical absorber layer is over-laid by either a Schottky barrier layer (possibly with an interposed lightly doped or intrinsic semiconductor layer) or an n-type semiconductor layer. The thickness and doping in the P-type segment of the window layer is designed such that when no bias is applied across the rectifying junction, at least on the order of 100 Angstroms of the P-type window layer remains undepleted. Electrical contacts are separately made to both the N or Schottky layer of the rectifying junction and P-type section of the cathode. Consequently, when no bias voltage is applied, the window layer of the instant invention functions in the same manner as the window layer described in U.S. Pat. No. 5,268,570. However, when a reverse bias of a few volts is applied, the depletion layer associated with the rectifying junction will extend into the optical absorber layer. The electric field associated with the depletion layer will prevent photoelectrons generated in said layer from diffusing to the vacuum surface. The applied bias voltage should be sufficient to pull the conduction band of the window layer below that of the undepleteted absorber layer. Similarly, any dark current electrons generated as a result of the applied field will be captured. These electrons may be temporarily captured in the well formed at the depleted hetero-junction. Eventually, these electrons will be thermally emitted over the hetero-junction barrier. Electrons that drift into the contact layer will be conducted away.